1. Field of the Invention
The present invention relates to the deposition of II-VI semiconductor films, and, more particularly, to the deposition of ternary chalcogenide semiconductor films, such as HgCdTe and HgZnTe, onto silicon substrates.
2. Description of Related Art
Ternary II-VI semiconductor films find use in many infra-red applications, such as in IR focal plane arrays (FPAs). Examples of such ternary II-VI semiconductor compounds include HgCdTe and HgZnTe, which are also known as chalcogenides.
In the past, ternary II-VI semiconductors were formed on ternary II-VI substrates, such as CdZnTe, which were then bonded to silicon substrates by indium bump technology. However, the thermal cycling to 77 K, which is the temperature used for detection, has tended to create reliability problems. It is desired to use silicon-based substrates rather than bulk CdZnTe substrates for large area focal plane arrays, since the read-out circuit chips are also silicon, and thus expansion and contraction with thermal cycling would be at the same rate.
A variety of approaches have been tried to form II-VI films on silicon. Prior methods of depositing II-VI films on Si substrates fall into two categories: (1) those employing an intervening buffer layer, usually GaAs or a Group II fluoride such as CaF.sub.2 or BaF.sub.2, between the Si substrate and the II-VI layer; and (2) those that deposit the II-VI film directly on the Si substrate with no intentional placement of any intervening layers. However, for the most part, various buffer layers have been used, due to the disparity of lattice parameters between silicon and the II-VI semiconductor film, which often is of the order of 15% to 20%.
Group II fluoride buffer layers on silicon have been used, with the II-VI film formed on the buffer layer; see, e.g., A. N. Tiwari, et al., "Heteroepitaxy of CdTe(100) on Si(100) Using BaF.sub.2 --CaF.sub.2 (100) Buffer Layers", Journal of Crystal Growth, Vol. 111, pp. 730-735 (1991). However, this approach is presently substantially inferior in crystalline quality to II-VI films deposited either on GaAs/Si or silicon.
Faurie and coworkers have deposited CdTe directly on silicon; see, e.g., R. Sporken, et al., "Current Status of Direct Growth of CdTe and HgCdTe on Silicon by Molecular Beam Epitaxy", Journal of Vacuum Science and Technology, B 10 pp. 1405-1409 (1992). However, the {111} orientation employed in their work has often resulted in extensive domain and lamellar twin formation that renders the material unreliable for use as alternative substrates in liquid phase epitaxy (LPE), molecular beam epitaxy (MBE), or metal organic chemical vapor deposition (MOCVD).
A discussion of the growth of CdZnTe layers on a GaAs buffer formed on a silicon substrate for large-area HgCdTe infra-red (IR) focal plane arrays (FPAs) is given by S. M. Johnson et al, "MOCVD Grown CdZnTe/GaAs/Si Substrates for Large-Area HgCdTe IRFPAs", 1992 HgCdTe Workshop, Boston, Mass., (Oct. 13-15, 1992). The drawback with using a substrate containing GaAs is that both Ga and As are electrically active dopants in HgCdTe and other II-VI materials. This creates a significant problem concerning the threat of contaminating an LPE HgCdTe melt with both Ga and As should a wafer fracture or uncontrolled etchback of the CdZnTe buffer layer occur. The melt would rapidly dissolve the exposed Ga and As, thus contaminating the melt. The threat of such a catastrophic event is a significant drawback to II-VI growth on GaAs-on-Si.
Further, silicon substrates with a buffer layer of GaAs are generally obtained from an outside vendor and cost approximately $2,000 (1993 dollars) for a 4-inch diameter wafer versus a cost of $20 for a comparable uncoated silicon wafer. Purchase of GaAs-on-Si substrates also requires reliance on the quality control protocols of two separate manufacturers--the silicon substrate supplier and GaAs epitaxial foundry.
Thus, a need remains for the formation of good quality ternary II-VI semiconductor films on a silicon substrate without the added complications imposed by the presence of an extraneous III-V interlayer.